The MERGE statement in COBOL is used for merging two or more sorted input files into a single sorted output file. In this scenario, it would be the appropriate choice to merge sales data from different regions sorted by date.

A breakpoint in COBOL is a condition set by the programmer that causes a temporary stop in program execution during debugging. This allows the programmer to inspect variables and control the flow of the program for effective debugging.

Runtime errors related to file handling in COBOL programs often stem from issues such as incorrect file permissions, invalid file paths, or file access conflicts. To debug this issue, one would start by checking the file permissions and access rights to ensure the program has the necessary permissions to read from or write to the file. Additionally, examining the file handling code, ensuring proper file open/close operations, and using file monitoring tools or file system debuggers can help identify and resolve file-related runtime errors in COBOL programs.

A structured error-handling approach in COBOL offers advantages such as improved code readability, ease of maintenance, and better program control. It enhances the overall quality of the program and makes it easier to manage exceptions in a systematic manner.

To optimize performance and avoid bottlenecks, it's crucial to consider the appropriate use of Record-level locks for different file access patterns. Record-level locks restrict access to individual records, allowing multiple users to work with other records concurrently, minimizing contention and improving overall system efficiency.

Duplicate records in COBOL file handling refer to those with identical key values. These records are distinguished based on a specified key field, and the duplicates must be managed to avoid data inconsistencies.

In inventory management, using the OCCURS clause with the DEPENDING ON phrase is suitable. It allows dynamic allocation of storage based on the value of a data item, enabling flexibility in handling varying stock levels for different items.

The VARYING clause in COBOL is used for handling variable-length records in files. It allows a field to have a variable length based on the actual data stored. This is particularly useful when dealing with records of varying sizes in file processing.

In COBOL, a national (Unicode) character data type is defined using the PIC N clause. This allows the representation of Unicode characters in the program, ensuring compatibility with international character sets.

The SHAREOPTION clause in COBOL is crucial when dealing with VSAM indexed files in a multi-user environment. It specifies the level of file sharing allowed among users. DENYWRITE, for example, prevents other users from writing to the file simultaneously, ensuring data integrity.

Dividing an integer by zero in COBOL results in an arithmetic exception. The program will encounter a runtime error due to division by zero, and appropriate error handling should be implemented to address such scenarios.

COBOL typically relies on the underlying operating system for record locking when multiple programs access the same file in "I-O" (Input-Output) mode. It doesn't inherently provide built-in record locking mechanisms.